In recent years, there has been an increasing demand for electronic devices capable of operation at high power levels and high temperatures. With a very high thermal conductivity (about 4.9 W/cm), high saturated electron drift velocity (about 2.7*107 cm/s) and high breakdown electric field strength (about 3 MV/cm), silicon carbide is a suitable material for high temperature, high voltage and high power application. These applications may e.g. imply blocking voltages in excess of 10 kV, conduction current greater than 100 A at a switch rate of more than 150 kHz. Such high power devices will require areas in excess of 1 cm2 and thick (about 100-200 μm) epitaxial layers, or epilayers, with good surface morphology and low defect density. Future devices might require even thicker layers. Hence, a manufacturing process facilitating a reproducible growth of thick, high quality layers at a high growth rate is most desirable.
It has been repeatedly shown that threading defects like micropipes, screw and edge dislocations in SiC commercial wafers are device performance limiting defects. Further, high-power devices normally requires low-doped (e.g. 1013-1015 cm−3) epitaxial layers.
Surface morphology of the epitaxial layers is an important characteristic since morphological defects may hamper device characteristic of the final semiconductor device. In addition, the epitaxial growth mode of SiC is determined by step-flow mechanism, and hence the growth of thick layers normally put high demands on morphology due to the inherent amplification of surface features, e.g. a pronounced enlargement of substrate imperfections such as polish-induced scratches, polytype inclusions and micropipes when conventional methods are used. Further, the high growth temperature, chemical stability, and the lack of congruent melting of SiC set limitations to the crystal growth methods. Attempts have been made to use High Temperature Chemical Vapor Deposition (HTCVD) for producing thick epitaxial layers without succeeding in achieving satisfactory high reproducibilities at sufficiently high growth rates.
An object of the inventions is to overcome or alleviate the above mentioned drawbacks.